PIC18F2450_08 Datasheet, PDF (26/324 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

PIC18F2450/4450
2.2.1
OSCILLATOR MODES AND
USB OPERATION
Because of the unique requirements of the USB
module, a different approach to clock operation is
necessary. In previous PIC^® microcontrollers, all core
and peripheral clocks were driven by a single oscillator
source; the usual sources were primary, secondary or
the internal oscillator. With PIC18F2450/4450 devices,
the primary oscillator becomes part of the USB module
and cannot be associated to any other clock source.
Thus, the USB module must be clocked from the
primary clock source; however, the microcontroller
core and other peripherals can be separately clocked
from the secondary or internal oscillators as before.
Because of the timing requirements imposed by USB,
an internal clock of either 6 MHz or 48 MHz is required
while the USB module is enabled. Fortunately, the
microcontroller and other peripherals are not required
to run at this clock speed when using the primary
oscillator. There are numerous options to achieve the
USB module clock requirement and still provide flexi-
bility for clocking the rest of the device from the primary
oscillator source. These are detailed in Section 2.3
“Oscillator Settings for USB”.
FIGURE 2-1:
PIC18F2450/4450 CLOCK DIAGRAM
OSC2
OSC1
Primary Oscillator
Sleep
XT, HS, EC, ECIO
T1OSO
T1OSI
Secondary Oscillator
T1OSCEN
Enable
Oscillator
PIC18F2450/4450
PLLDIV<2:0>
USB Clock Source
÷ 12
111
÷ 10 ₁₁₀
÷ 6 ₁₀₁
÷ 5 ₁₀₀
÷4
011
÷3
010
USBDIV
(4 MHz Input Only)
0
96 MHz
PLL
÷2
1
÷2
001
÷ 1 ₀₀₀
HSPLL, ECPLL,
XTPLL, ECPIO
CPUDIV<1:0>
FSEN
1
USB
Peripheral
CPUDIV<1:0>
÷ 4 ₁₁
÷3
10
÷ 2 ₀₁
÷ 1 ₀₀
÷6
÷4
11
÷ 4 ₁₀
÷3
01
÷ 2 ₀₀
1
0 Primary
Clock
FOSC3:FOSC0
0
CPU
IDLEN
Peripherals
T1OSC
OSCCON<6:4>
Internal Oscillator
Internal RC Oscillator
31.25 kHz
Clock
Control
FOSC3:FOSC0 OSCCON<1:0>
Clock Source Option
for Other Modules
WDT, PWRT, FSCM
and Two-Speed Start-up
DS39760D-page 24
© 2008 Microchip Technology Inc.